you're right, you don't know what's going on. the apple hides itself if the space is needed by menus. see, on the macintosh, that "white bar" is called the menu bar. the menu bar holds menus. the menus displayed are those of the frontmost process. i dont know what YOU want to use the space for, but the apple certainly doesn't conflict with anything that is supposed to be there (menus).
As stated that white bar is the menubar, Mac programs share one menu bar located at the top of the screen. At the same time I do feel that the Apple logo can be done away with, it is one of those things that new users are going complain about when clicking on it results in no action whatsoever.